1. Field of the Invention
The present invention concerns a completely wireless dual-access test fixture for interfacing a printed circuit board with a board tester.
2. Description of the Related Art
A test fixture is an apparatus designed for mounting and interfacing a printed circuit board (PCB) to be tested by a test and analysis apparatus, otherwise referred to as a board tester. In general, there are two basic types of testers, in-circuit testers for identifying or verifying electrical shorts, open-circuits, component tolerances and IC chip "clocking", and functional testers for verifying the appropriate functional operation of the PCB. An example of an in-circuit tester is the GENRAD 2272/2282 board tester. The present invention is primarily concerned with fixtures for in-circuit testers, although the invention may be equally applied to test fixtures for functional testers.
The purpose of a dual-access test fixture is to access test points or pads simultaneously on the top and bottom sides of the PCB under test. As the size of PCBs get smaller and become more densely populated with components, the test pads on the PCB consequently decrease in size. These smaller test pads require greater accuracy and much tighter tolerances of the fixture apparatus, since otherwise good PCBs would fail because of misalignment of the probes or pins with the pads for electrical interfacing. Furthermore, as the PCBs under test become more sophisticated, it is desirable and often necessary to test separate circuits on the bottom and top sides of the PCB. Several dual-access test fixtures have been implemented to solve some of the ever-increasing alignment problems, but many alignment problems still exist, especially due to the advent of smaller test pads.
The existing test fixtures have several deficiencies causing an inordinate percentages of failures. One major problem is the use of electrical wires in one form or another to complete electrical connections. The wires are typically in the form of wirewrap, where long, individual wires are used for each test connection, or ribbon cable coupled between the top and bottom fixtures and mounted to corresponding connectors.
Many false signal and noise problems arose when using wirewrap or ribbon cables to electrically connect test pins used to contact the test pads of the PCB under test. The wires add stray capacitance or inductively couple signals in nearby test wires, causing electrical noise and false signals. This problem has not been previously solved, so that the only method to achieve an accurate test on a high speed PCB was to reduce the frequency of the test. Frequency reduction is undesirable, since the test is not completely reliable at slower speeds. The most accurate and reliable test is performed at the true clock frequency.
U.S. Pat. No. 5,157,325 ('325), issued Oct. 20, 1992 to Murphy, discloses an existing version of "wireless" fixture technology, which is hereby incorporated by reference. In the so-called wireless technology of the '325 patent, specially designed test probes comprising doubled-ended pogo pins were mounted to a probe plate, where one side of the test pins would electrically contact or engage the test pads of the PCB under test, and where the opposite end of the test pins would electrically engage test pads on the surface of an interface PCB (IPCB). Two probe plates, each mounted with pogo pins, were provided on either side of the PCB under test. The other ends of the pogo or test pins on the bottom probe plate would electrically engage test pads on a bottom IPCB. The bottom IPCB also had rows of input/output (I/O) pads on its bottom side to electrically interface with I/O pins of the tester unit itself. Corresponding test pads formed on the upper and lower surfaces of the bottom IPCB were electrically connected together through conductive traces on and within the bottom IPCB. Thus, electrical connections were made between the test pads located on the bottom side of the PCB under test and the tester through the pogo pins on the bottom probe plate and the bottom IPCB.
On the top side of the PCB under test, the top probe plate was mounted with similar double-ended pogo pins, which were used in a similar manner to electrically engage test pads on the top side of the PCB under test to corresponding test pads on a top IPCB. The test pads on the top IPCB were electrically connected to a connector mounted on the top IPCB using routed traces on the top IPCB. One end of a ribbon cable was plugged into the connector, and the other end of the ribbon cable was plugged into a similar connector mounted on the bottom IPCB. The test fixture apparatus of the patent '325 was not a true wireless system due to the use of the ribbon cable. The ribbon cable also caused other problems. For example, when closing the fixture for testing, the ribbon cable would often get caught between the top and bottom frames, and would consequently break and need replacement. Worse yet, the internal wires would often be severed, resulting in a hidden defect which was very difficult to detect or debug. Further, breakage of the ribbon cable sometimes caused electrical shorts increasing the likelihood of electrical shock.
The previous dual-access wireless fixture described in the '325 patent used a handler having a separate top access "cassette" containing the top fixture assembly, and a separate bottom frame containing the bottom fixture assembly. The handler caused severe alignment problems resulting in an inordinate number of failures, damage to the PCB under test and also damage to the fixture itself. In particular, when using the old handler in the dual-access application, the top cassette was able to shift or slide relative to the bottom frame before vacuum was applied, causing misalignment and broken test probes. Although guide pins and bushings were used, they were not pre-aligned before vacuum was applied. When vacuum was applied, the top access cassette was forced into alignment while the test pins were in contact with the test pads of the PCB. Such forced shifting and alignment reduced the lifetime of the guide pins and bushings, and the test pins and test pads. Several adjustments and test cycles were likely for each test, resulting in greater test cycle times. The necessary adjustments sometimes involved manually shifting the upper cassette to force alignment, often resulting in damage to the PCB under test, the test pins or other parts of the fixture.
One inherent alignment problem with the existing dual-access fixture is that the top access cassette was pivoted with respect to the bottom frame along a common axis to close the fixture prior to testing. The top cassette was lowered on top of the PCB under test and bottom frame in a similar manner as a hood on a car or the lid of a top-loading washing machine. When vacuum was applied, the top access was forced further downwards, resulting in lateral movement due to the single pivot axis. Since the test pins would slide along the test pads of the PCB under test due to the lateral movement, the pins would undergo wiping action from the point where the pins first contacted the pads until its final test position. This sliding and scraping often caused bent or broken test probes and damage to the test pads on the PCB. Furthermore, many of the pins were often either mis-aligned, barely contacting the test pads, or not contacting the test pads at all.
Other methods were attempted to solve the inherent alignment problems. For example, parallel linkage techniques have been applied to raise and lower the top access relative to the bottom frame, using two or more links similar to the compartments in a tackle-box. Nonetheless, the mounting apparatus was still designed to pivot further to complete the alignment, so that the pivot motion, however slight, still occurred after vacuum was actuated causing significant lateral movement of the pins relative to the pads. The lateral movement caused stress on guide pins and bushings and further caused a wiping action of the test probes. These alignment problems always shortened cycle life of the fixture.
Another problem was tolerance stack-up, caused by separate spacers stacked in different layers of the fixture to achieve an aggregate distance. For example, one set of spacers would be used between the bottom IPCB and an alignment plate, and a separate set of spacers between the alignment plate and the probe plate to achieve an over-all aggregate distance between the bottom IPCB and the bottom probe plate. This distance is critical since it defines the compression ratio of the probe pins used to electrically engage test pads on the bottom IPCB. The separate spacers caused deformation of the alignment plate and the bottom IPCB, resulting in a non-uniform distance between the probe plate and bottom IPCB. Thus, the test pins were not uniformly compressed, so that some were compressed only slightly thereby compromising electrical integrity, while others were over-compressed thereby causing damage to the test pads or test pins.
It is desirable, therefore, to provide a dual access fixture without wires and with significantly improved alignment techniques. Eliminating the wires would allow test at full speed and reduced false signals, while improved alignment would reduce stress and breakage of the test pins and test pads.